Summary: Live vaccines produce better and longer-lasting anti-tuberculous immunity than killed or sub-unit vaccines. Live BCG is the only licenced tuberculosis vaccine in the US but does little to protect adults against this highly infectious disease. Mice vaccinated with a sublethal dose of virulent M. tuberculosis H37Rv develop high levels of resistance against an aerogenic challenge with M. tb. Erdman given 1 to 3 months later. On the other hand, the avirulent variant,H37Ra induces little or no immunity against this challenge. Virulence genes transferred from H37Rv to H37Ra by electoroporation could restore the protective ability of this avirulent recombinant, enabling it to grow and survive within the spleens of vaccinated animals without causing clinical symptoms of disease. Such vaccinated mice, challenge 1 - 2 months later with a small aerogenic inoculum of M. tb Erdman showed reduced growth of the virulent organisms in the lungs and spleen and the rate of spread by the lung infection to the spleen are prolonged, compared to that seen in control mice vaccinated with live BCG or the empty vector control. M. smegmatis is a non-pathogenic rapid grower widely used in biochemical and genetic studies. Recombinant strains containing large fragments of DNA harvested from virulent M. Tb Erdman were introduced into naive C57BL/6 mice by the intravenous, subcutaneous, intranasal and aerogenic routes and the growth was followed for up to 1 month. All of the mice survived, even when challenged with massive doses of the recombinant. The rM. smegmatis failed to grow in vivo and was rapidly eliminated from the lungs and spleen. Challenge of the vaccinated mice indicated that their resistance to M. tb was no better than the unvaccinated controls.